1. Field of the Invention
The present invention generally relates to a laser beam isolation apparatus of a laser printer, and more particularly, to a laser beam isolation apparatus of a laser printer which prevents a laser beam from being emitted out of a laser scanning unit (LSU) when the LSU erroneously operates in the absence of a developer unit of the laser printer due to abnormality of related parts.
2. Description of the Related Art
A general conventional printing apparatus, a laser printer in this instance (FIG. 1), includes a paper feeding portion 1 in which paper sheets are stacked, a paper conveying portion 2 to convey a paper sheet from the paper feeding portion 1, and a laser scanning unit (LSU) 4 to form an electrostatic latent image on a photoreceptor drum 3. The apparatus further includes a developer unit 6 to develop the electrostatic latent image on the photoreceptor drum 3 into a visible image with a supply of toner thereon and then forming, with the supply of a transfer voltage between a transfer roller 5 and the photoreceptor drum 3, a toner image on the paper sheet being conveyed by the paper conveying portion 2, a fusing portion 7 to fix the toner image on the paper sheet with heat and pressure, and a paper discharging portion 8 to discharge the paper sheet with the toner image fixed thereon.
Among the above parts of the printer, the LSU 4 functions to form an electrostatic latent image on the photoreceptor drum 3 in accordance with an image signal by irradiating a laser beam 11 onto the photoreceptor drum 3, and includes a laser diode 4a to emit the laser beam 11, a rotary polygon mirror to deflect and reflect the laser beam 11 from the laser diode 4a at a constant linear velocity, a scan lens 4c to compensate for an error included in the laser beam 11 reflected from the rotary polygon mirror 4b, and a reflective mirror 4d to reflect the laser beam 11 towards a surface of the photoreceptor drum 3. The laser beam 11 passes through a laser beam passing hole 4e. 
Generally, the LSU 4 is provided above the developer unit 6 and emits a laser beam 11 onto the surface of the photoreceptor drum 3 via beam passing holes 4f of the developer unit 6. This construction causes undesirable exposure of the user to the laser beam 11 when he/she replaces an old developer unit 6, or removes the developer unit 6 to check a paper jam.
In order to protect the user from a possible exposure to the laser beam, conventionally, the printer was provided with a laser beam isolation switch 10 as shown in FIGS. 2A and 2B, which cuts off the power supply of the laser diode 4a upon opening the cover 20 relative to the hinge axis 20a for repair, replacement, or the like.
The laser beam isolation switch 10 includes a switch 23 formed in a housing 14 and connected to the power supply for the LSU 4, a switch operating member 60 formed in the housing 14 and operating the switch 23 in accordance with the opening and closing of the cover 20, and a projection 50 formed on the cover 20 and movable in association with the switch operating member 60 to operate the switch 23 by pressing the switch 23. The switch operating member 60 includes an operating portion 61 contacting the projection 50, a switch contacting portion 63 formed at a predetermined angle relative to the operating portion 61 to operate the switch 23 by contact, a hinge portion 62 arranged between the operating portion 61 and the contacting portion 63 to guide the rotational movement of the switch operating member 60, and a spring 64 disposed on the housing 14 to elastically pull the switch contacting portion 63.
Describing the operation of the conventional laser beam isolation switch 10 in detail, first, with the closing of the cover 20 as shown in FIG. 2A, the operating portion 61 of the switch operating member 60 is pressed downwards by the projection 50. Accordingly, the switch contacting portion 63 of the switch operating member 60 is rotated about the hinge portion 62 counterclockwise against the recovering force of the spring 64, thereby pressing the switch 23. As a result, the laser diode 4a is operated normally, and emits laser beam 11.
Next, with the opening of the cover 20, the projection 50 is spaced apart from the operating portion 61, followed by the switch contacting portion 63 rotated by the recovery force of the spring 64 about the hinge portion 62 clockwise to subsequently release the switch 23. As a result, operation of the laser diode is stopped, and the laser beam 11 is not released.
However, with the conventional laser beam isolation switch 10 as described above, the operation of the laser diode 4a is not stopped if the cover 20 is opened for the developer unit 6 replacement with the switch 23 not being operated due to an internal short circuit. As a result, the user is exposed to the laser beam emitted from the LSU 4. Exposure to the LSU 4 can be dangerous especially if the laser beam 11 is directly emitted from the LSU 4 to parts of the body, such as an eye, when the user opens the cover 20 and removes the developer unit 6.
In an attempt to solve the above problems, a laser printer having a laser beam isolation apparatus 51 as shown in FIGS. 3A and 3B, which covers a laser beam passing hole 4e40 of the LSU 4′ so as to block the laser beam 11′ that can be irradiated from the LSU 4′ during a removal of the developer unit 6′.
As shown in FIG. 3A, the laser beam isolation apparatus 51 of the laser printer includes a blocking plate 52 in a flattened U-shape movably secured to the axis 54 with one end and the other end to pivot to open and close the laser beam passing hole 4e′ formed at a lower side of the housing of the LSU 4′, an elastic spring 53 disposed between the housing of the LSU 4 and the blocking plate 52 to elastically support the blocking plate 52 to the blocking position (see FIG. 3B) where the blocking plate 52 blocks the laser beam passing hole 4e′, and an operation projection 55 to maintain the blocking plate 52 at an opening position where the blocking plate 52 opens the laser beam passing hole 4e′ during a mounting of the developer unit 6′.
With the developer unit 6′ being mounted under the LSU 4′ as shown in FIG. 3A, the operation projection 55 of the developer unit 6′ pushes the blocking plate 52 towards the opening position. Accordingly, the laser beam 11′ is emitted from the LSU 4′ onto the photoreceptor drum 3′ through the laser beam passing hole 4e′ of the LSU 4′ and through the passing hole 4f′ of the developer unit 6′.
Then, with the cover 20′ being opened and the developer unit 6′ being removed from the LSU 4′ as shown in FIG. 3B, the blocking plate 52 is returned to the blocking position by the recovering force of the elastic spring 53. Accordingly, the blocking plate 52 is moved to close the laser beam passing hole 4e′ formed at a lower side of the LSU 4′ by the recovering force of the elastic spring 53. The laser beam 11′ from the LSU 4′ is reflected inwards of the housing of the LSU 4′ from the lower side 52′ of the blocking plate 52, and thus, the user is not exposed to the laser beam 11′.
While the conventional laser beam isolation apparatus 51 effectively blocks the laser beam 11′ of the laser diode (not shown) from being emitted outside of the LSU 4′ due to abnormality of the related parts during removal of the developer unit 6′, the structure of the blocking plate 52 requires the employment of the elastic spring 53 to move the blocking plate 52 to the blocking position. As additional elastic springs 53 are required to move the blocking plate 52 to the blocking position, the structure of the laser beam isolation apparatus 51 becomes complex, and manufacture costs increase.
Further, in the conventional laser beam isolation apparatus 51, foreign substances such as dust sometimes enter into the interior of the LSU 4′ through the laser beam passing hole 4e′ with the movement of the blocking plate 52 to the blocking position, thus deteriorating the performance of the LSU 4′.
In order to prevent entrance of foreign substances through the laser beam passing hole 4e′, the laser beam passing hole 4e′ may be sealed by a transparent glass. However, this has a drawback of high manufacturing cost due to employment of additional parts, i.e., the transparent glass.